The present invention relates to elastomeric block copolymers star-shaped molecular architecture, where the star-shaped molecular architecture has at least two different arms of the star, where at least one arm of the star is composed of at least one block A which forms a hard phase and which comprises copolymerized units of at least one vinylaromatic monomer, and optionally of at least one block B which forms a first elastomeric (soft) phase and which comprises a, or a plurality of various, diene monomer(s), and of at least one elastomeric block B/A which forms a soft phase and which comprises copolymerized units not only of at least one vinylaromatic monomer but also of at least one diene, to a process for producing the elastomeric block copolymers of the invention, to the use of the elastomeric block copolymers of the invention for producing moldings, such as foils, foams, thermoformed moldings, injection moldings; or of flexible tubing or extruded profiles; or as adhesive layer in multilayer foils; as seal; as adhesion promoter or thermoplastic component in wood-plastics composites; as hot-melt adhesive component; for impact-modifying thermoplastics or elastomers, or for compatibilization in polymer mixtures, and also to foils, foams, thermomoldings, injection moldings, flexible tubings, or profile extrudates composed of at least one elastomeric block copolymer of the invention.
Block copolymers of vinylaromatics (e.g. styrene) and dienes (e.g. 1,3-butadiene) are copolymers made of a plurality of polymer molecule regions (known as blocks) which have relatively uniform internal structure and which have been arranged in series or have been linked in some other way. As a function of structure and content of diene monomers, at a certain temperature the overall properties of these materials can be elastomeric or rigid and non-elastomeric; the overall behavior of these materials in relation to their environment is therefore either elastomeric and similar to that of a polydiene, an example of an important material being that known as SB rubber, or is similar to that of transparent, impact-resistant styrene polymers. Conventional terminology is analogous to that used for impact-modified polystyrene, and those portions of the molecule which determine the elastomeric behavior are therefore termed soft phase, and the rigid portions of the molecule (generally the straight polystyrene fraction) are generally termed hard phase. SB rubbers cannot be processed as thermoplastics but instead, like conventional diene polymers, have to be vulcanized for use; this severely restricts their use and increases processing cost.
The present invention relates in general terms to transparent elastomeric block copolymers which can be processed by purely thermoplastic methods and which comprise vinylaromatics and dienes, and which have elastomeric behavior and excellent mechanical properties, and which can be produced with good space-time yields.
The following preliminary comments should be noted:
In the anionic polymerization reaction that leads to what are known as living polymers, the growth of a chain molecule takes place at a chain end which theoretically, in the absence of spontaneous chain-termination reaction or chain-transfer reaction, remains living (capable of polymerization) for an indefinite time, and the reaction of the living polymer with mono- or polyfunctional reactants provides a versatile method for constructing block copolymers, although the selection of monomers is subject to restriction. Materials that have achieved industrial significance are in essence block copolymers involving firstly vinylaromatic compounds, i.e. styrene and its derivatives, and secondly dienes, in particular butadiene or isoprene. Block copolymers are obtained by almost complete polymerization of each charge of monomer, and then changing the monomer(s). This procedure can be repeated many times.
Linear block copolymers are described by way of example in U.S. Pat. Nos. 3,507,934 and 4,122,134. Star-shaped block copolymers are described by way of example in U.S. Pat. Nos. 4,086,298, 4,167,545 and 3,639,517.
The property profile of these block copolymers is in essence a function of the content of copolymerized diene monomers, i.e. length, arrangement, and quantitative proportion of polydiene blocks and of polystyrene blocks. An important part is also played by the nature of the transition between different blocks. WO 95/35335 A1 provides a detailed description by way of example of the effect of sharp and what are known as tapered transitions (as a function of whether the change of monomer takes place abruptly or gradually).
In the case of block copolymers with tapered block transition, the distribution of sequence lengths is certainly not random, but instead there is a shift in the sequence length of the straight diene phase in relation to the polystyrene phase, and therefore of the mass ratio, in favor of the diene phase. A disadvantage of this is that the adverse properties of the diene polymers are unnecessarily emphasized in the behavior of the material during processing, while the styrene-rich end of the block impairs elastomeric properties.
The property profile (toughness, transparency, gas permeability) of materials having more than 35% by weight diene content would make them particularly suitable for medical-technology applications, such as infusion tubing and infusion drip chambers, and extensible foils, but they are very difficult to process by profile extrusion, injection molding, or blown-film extrusion. Despite stabilization with antioxidants and free-radical scavengers, they are highly heat-sensitive and tend to stick, and complicated procedures using additives are therefore necessary. Processing via extrusion or injection molding can be rendered completely impossible by what is known as blocking (adhesion of foils and flexible tubing on the roll), poor demoldability, and the tendency toward thermal crosslinking at the conventional processing temperatures.
In this connection, WO 95/35335 A1 proposes inserting, in a vinylaromatic-diene block copolymer made of blocks which form a hard phase (block type A) and of blocks which form a soft phase, in place of a straight polydiene block as soft phase, a B/A block made of diene units and of vinylaromatic units and having relatively random structure. The structure here can be, on statistical average, homogeneous or inhomogeneous along the chain. This method gives elastomeric block copolymers which can easily be produced on a large industrial scale and which have a maximum of toughness at low diene content and moreover can easily be processed like thermoplastics in extruders and injection-molding machines. These elastomeric block copolymers themselves represent a considerable advance over the styrene-rich block copolymers known hitherto having tapered block transitions, and over the diene-rich block copolymers having one or more straight diene blocks, where the diene used usually comprises 1,3-butadiene or isoprene.
DE 196 15 533 A1 says that, in order to mitigate gelling which can nevertheless occur under prolonged thermal stress and shear stress when the elastomeric block copolymers of WO 95/35335 A1 are extruded, the block (B/A) made of diene units and of vinylaromatic units has a strictly random structure where the relative proportion of 1,2-linkages of the polydiene, based on the total of 1,2- and 1,4-cis/trans linkages is always below about 12-15%. This can be achieved by producing the copolymers of vinylaromatics and of dienes via polymerization in the presence of a potassium salt soluble in nonpolar solvents.
The elastomeric block copolymers described in WO 95/35335 A1 and DE 196 15 533 A1 can still be further improved in respect of their resistance to shear.